Method of producing an undercut, concave, self-contained bearing surface

ABSTRACT

The invention relates to a method of producing concave, self-contained bearing surfaces, which are at least part of spherical indentations which extend beyond the sphere equator, by means of machining involving the removal of material of pre-formed indentations in workpieces by means of a tool having a tool cutting edge orbiting around a tool axis of rotation with an orbit diameter which is equal to the length of a chord extending up to a deepest point of the bearing surface, whereby the cutting movement is made up of the orbital movement of the tool cutting edge and a rotational relative movement between workpiece and tool around a vertical axis through the sphere center, and the feed movement is a movement of the tool relative to the workpiece along the tool axis of rotation which is obliquely set at the same angle of inclination relative to the vertical axis as is the chord set to the sphere equator. For reducing manufacturing times and increasing the dimensional exactitude of the bearing surface it is proposed in accordance with the invention that the chord (S) originates from the sphere equator (A), and the regions of the indentation (7) to the two sides of the sphere equator (A) are machined with two separate feed movements. The invention further relates to a bearing surface as product of this method.

The invention relates to a method of producing an undercut, concave andself-contained bearing surface in accordance with the preamble of claim1.

This method is known as so-called spinner method, which is employed inthe manufacture of axial and radial piston machines for forming hollowsphere segments in stroke-generating bodies intended for mounting ballend pins at displacement bodies or slippers, which stroke-generatingbodies are for example the drive disks of bent axis machines, slippersor displacement bodies, such as for example pistons. The concave bearingsurface of each hollow sphere segment extends beyond the equator of thesphere; its region located above the equator thus being undercut andassuming, for example in the case of the mounting of a piston, thenecessary piston return function.

The diameter of the tool cutting edge orbit is equal to the length of achord which extends from the upper limiting edge of the roundedindentation-like concave bearing surface to its lower apex. By feedingthe tool, introduced into the indentation, along its obliquely set toolaxis of rotation, set at the same angle of inclination to the verticalaxis as the chord to the upper limiting edge or sphere equator, so farthat the cutting edge orbit impinges upon the upper limiting edge of thebearing surface and therewith also on its lower apex, the desired hollowsphere segment is formed.

As a consequence of the angle of inclination of greater than 45°,determined through the size of the undercut, which the tool axis ofrotation encloses with the vertical axis, there is required a relativelythin tool mounting shaft which allows only a correspondingly smallloading and feed speed of the tool, since otherwise the necessary highworking precision cannot be ensured. This means that the manufacturingtimes are undesirable long and it is not certain that the necessarydimensional exactitude of the bearing surface can be attained.

It is the object of the invention to so further develop the methodmentioned in the introduction that the manufacturing time can be reducedand the dimensional exactitude of the undercut bearing surface can beincreased.

This object is achieved in that the chord, which determines the diameterof the cutting edge orbit, extends from the sphere equator, and in thatthe region of the indentation below the sphere equator and that abovethe sphere equator are machined one after the other with two separatefeed movements.

In this way, when determining the diameter of the cutting edge orbit,the undercutting of the bearing surface is not taken into consideration,so that the tool axis of rotation is, during the feed, obliquely setwith an angle of inclination with respect to the vertical axis which isequal to or smaller than 45°. Correspondingly, the tool mounting shaftcan be formed more strongly so that it can withstand greater loading andthus higher feed speeds of the tool and therewith makes possible shortermanufacturing times with higher dimensional accuracy. The diameter ofthe cutting edge orbit determined in accordance with the invention, nottaking into account the undercutting of the bearing surface, makes itnecessary to produce the two bearing surface regions below and above thesphere equator in two separate feed movements which are preferablycarried out with the tool axis of rotation in the same disposition, thatis along the same feed axis, in opposite directions. The extra timeneeded for these two feed movements is, in comparison with the priorart, very small and is more than compensated for by the time gainafforded by the higher feed speeds.

For the production of the two bearing surface regions to the two sidesof the sphere equator with the same radius R, the tool introduced intothe indentation is expediently moved more deeply into the indentationwith one the two feed movements along the inclined tool axis of rotationuntil it takes up an end position, upon completion of the bearingsurface region below the sphere equator, in which end position the planeof the cutting edge orbit contains the chord of the sphere with theradius R; then, the tool is moved with the other feed movement havingopposite direction until it takes up a further end position, uponcompletion of the bearing surface region above the sphere equator, inwhich further end position the plane of the cutting edge orbit containsa further chord parallel to the first-mentioned chord and with the samelength in the sphere with the radius R.

In accordance with a further development of the invention, in additionto the feed movement along the tool axis of rotation carried out duringthe production of the bearing surface region above the sphere equator,the tool is moved in a further feed movement perpendicularly to thesphere equator in a direction towards the outside of the indentation. Inthis manner there can be produced a bearing surface the bearing surfaceregions of which to the two sides of the sphere equator have like radiiwhich extend from two middle points lying on the vertical axis,displaced one to the other. Thereby there is provided the necessary playin the bearing surface region above the sphere equator for thefunctioning of the ball joint formed by the concave bearing surface andthe ball end pin, in particular for the formation of a lubrication film,without adversely affecting the functioning of the bearing surfaceregion below the sphere equator, the radius of which can be so adaptedto the ball end pin that in this region there prevails a play which isas small as possible.

The necessary play in the bearing surface region above the sphereequator can also be attained with a bearing surface the bearing surfaceregion of which above the sphere equator has a radius which is largerthan the radius of the bearing surface region below the sphere equator.This larger radius is expediently produced in that the tool is moved,with the other of the two feed movements along the tool axis of rotationobliquely set with the angle of inclination, with a correspondinglygreater feed than in the case of the production of the above-mentionedbearing surface region with the same radius to the two sides of thesphere equator, so that upon completion of the bearing surface regionabove the sphere equator the tool takes up a further end position inwhich the plane of the cutting edge orbit contains a further chord,parallel to the first-mentioned chord, of the same length which runs ina sphere having a radius which is greater than the radius of the spherehaving the first-mentioned chord.

If the bearing surface takes up the entire spherical indentation thechord determining the diameter of the cutting edge orbit connects thesphere equator with the lower apex of the spherical indentation. A chordrunning in this way can be employed also for the case that the bearingsurface takes up only a part of the spherical indentation, i.e. isformed as a spherical zone open at its lower boundary edge. However,with such a spherical zone open at its lower boundary edge, it is morefavourable to connect with the chord a point of this lower boundary edgewith a point on the sphere equator lying diametrally opposite, in orderin this way to attain a lesser oblique positioning of the tool axis ofrotation relative to the vertical axis, i.e. to make possible a strongertool machining shaft.

Further features and advantages of the invention are apparent from theremaining subclaims.

Below, the invention is described with reference to a preferredexemplary embodiment and with reference to the single attached FIGUREwhich shows a tool approaching a workpiece, in vertical section.

The workpiece 1 partly illustrated in the FIGURE is the drive disk of abent axis machine. There are to be formed in this drive disk a number ofhollow sphere segments, having a radius R, for bearing ball end pins(not shown), which are located at the free ends of the pistons of thebent axis machine. The concave bearing surfaces 2 of the hollow spheresegments are undercut, i.e. they extend beyond the respective sphereequator A, so that they ensure the return movement of the piston. Thebearing surface region above the sphere equator A is indicated with thereference sign 3 and the region below the sphere equator is indicatedwith the reference sign 4. In the lower apex point P of the bearingsurface 2 there is formed a centring bore 5 which determines the lowerlimiting edge 6 of the hollow sphere segment and the disposition of avertical axis V running perpendicularly of the sphere equator A throughthe sphere centre point M.

The hollow sphere segments are prepared as accurately as possible on thecircular arc of the pistons with a form drilling and boring tool (notshown) with the formation of indentations 7, each with the centring bore5. Each indentation 7 has an upper, cylindrical surface part 8 whichextends to below the sphere equator A and a lower surface part 9 in theform of a spherical zone. The upper edge of the upper, cylindricalsurface part 8 is connected with the upper surface 11 of the workpiece 1via a chamfer 10 and represents the upper limiting edge 12 of theconcave bearing surface 2.

The lower surface part 9 of the indentation 7 and the concave bearingsurface 2 have each the form of a spherical indentation, if no centringbore is employed.

Each indentation 7 is to be transformed, through machining with theremoval of material, by means of a tool 14 rotatable around a tool axisof rotation 13, into a hollow sphere segment. The tool 14, merelyschematically illustrated, is mounted on a rotatable tool mounting shaft15, rotatable with the aid of a rotary drive (not shown), thelongitudinal middle axis of which shaft forms the tool axis of rotation13. It has a tool cutting edge 16 with a cutting part at the end face asthe main cutter and an auxiliary cutter on the circumference. Thediameter d of the cutting edge orbit U is greater than the diameter ofthe tool mounting shaft 15 and is so chosen that it is equal to thelength of a chord S which extends from an arbitrary point P_(a) on thesphere equator A up to a diametrally opposite point P_(b) on the lowerlimiting edge 6.

The workpiece 1 is so mounted on a rotary table (not shown) rotatablearound a vertical axis of rotation, by means of a mounting device(likewise not shown), that the circular arc on which the indentations 7are arranged cuts the axis of rotation of the rotary table. The mountingdevice may be rotated in steps around the centre of the circular arcrelative to the rotary table, in order in each case to bring one of thecentring bores 5 of the indentations 7 into congruence with the verticalaxis of rotation of the rotary table. As soon as this takes place therotary table and thus the workpiece 1 are caused to rotate, for exampleclockwise.

The tool 14 is mounted, above the workpiece 1, in a mounting device (notshown), which is pivotable around a horizontal pivot axis andtraversable horizontally and vertically, by means of its tool mountingshaft 15 and is, by means of corresponding pivoting relative to thevertical axis V defined by means of the centring bore 5 and coincidingwith the rotational axis of the rotary table, obliquely set with anangle of inclination a which is equal to the angle which the chord Smakes with the sphere equator A. The tool 14 is set in rotation, incounter-clockwise direction, by means of a drive (not shown) and thenmoved so far along its tool axis of rotation 13 until the point ofintersection Sp of the orbit U of the tool cutting edge 16 with the toolaxis of rotation 13 lies on the vertical axis V, as is shown in theFIGURE. Then, the tool 14 is moved into the indentation 7 along thevertical axis V until the point of intersection Sp lies on the sphereequator A, i.e. until the cutting edge orbit U coincides with the chainline L in the FIGURE.

In order now to produce the bearing surface region 4 below the sphereequator A, by machining with the removal of material, the tool 14 ismoved with a first feed movement along its tool axis of rotation 13obliquely downwardly as far as a first end position illustrated by thechain line S in the FIGURE, in which the chord S lies in the plane ofthe cutting edge orbit U. The feed a is equal to the radial spacing ofthe chord S from the sphere centre M.

Then, the tool 14 is moved by means of a second feed movement with thefeed a along the tool axis of rotation 13 directed oppositely to thedirection of the first feed movement, so far that its cutting edge orbitU again touches the sphere equator A and thus takes up the second endposition illustrated in the FIGURE by means of the chain line S', inwhich the plane of the cutting edge orbit U contains a second chord S'running parallel to the first-mentioned chord S and with the same lengthin the sphere with the radius R. The hollow sphere segment is producedwith the desired radius R=1/2 (r² /h+h), where r is the radius of thecutting edge orbit U and h the spacing of the chord S from the bearingsurface 2, measured on the middle perpendicular to the chord S.

During the second feed movement, the tool can be moved in verticaldirection upwardly and in this way the centre point M of the bearingsurface region 3 having the radius R above the sphere equator A can becorrespondingly displaced relative to the middle point M of the bearingsurface region 4 having the same radius R below the sphere equator.

On the other hand, by means of movement of the tool during the secondfeed movement, with a feed which is somewhat greater than theabove-mentioned feed a, the bearing surface region 3 above the sphereequator A can be produced with a radius which is correspondingly largerthan the radius R.

After the concave bearing surface 2 has been produced in this way, thetool 14 is withdrawn from the hollow sphere segment. By means of steppedturning of the mounting device relative to the rotary table, the centrebore 5 of the next indentation 7 is brought into coincidence with theaxis of rotation of the rotary table; the above described process stepsare now repeated.

The cutting movement during both feed movements is made up of theorbital movement of the tool cutting edge 16 and the rotational movementof the workpiece 1 around the vertical axis V. The same cutting movementcan be attained with a stationary workpiece 1 by means of a gyroscopicmovement of the tool 14 around the vertical axis V whilst maintainingthe angle of inclination α.

In contrast to the above-described procedure, the bearing surface region3 above the sphere equator A may be produced first and then that bearingsurface region below the sphere equator A.

The introduction of the tool 14 into the indentation 7 need notnecessarily take place along the vertical axis, but can also be effectedparallel to the vertical axis.

I claim:
 1. Method of producing an undercut, concave and self-containedbearing surface (2), which is at least part of a spherical indentationhaving an at least approximately constant radius (R), which indentationextends beyond a sphere equator (A) of the spherical indentation bymeans of machining with the removal of material of a pre-formedindentation (7) in a workpiece (1) by means of a tool (14) having atleast one tool cutting edge (16) rotating around a tool axis of rotation(13), defining a plane encompassing the orbit (U) of the tool cuttingedge which has a diameter (d) which is equal to the length of a chord(S) extending up to a deepest point (P_(b)) of the bearing surface,whereby a cutting movement of said tool is made up of the orbitalmovement of the tool cutting edge and a rotational relative movementbetween the workpiece and the tool around a vertical axis (V) extendingperpendicularly of the equator of said spherical indentation through thecenter (M) thereof, and wherein a feed movement of the tool is amovement of the tool relative to the workpiece along the tool axis ofrotation which is obliquely set at a same angle of inclination (α)relative to the vertical axis as the plane of the cutting edge orbit isset to the sphere equator (A), characterized in that,the diameter (d) ofthe plane of the cutting edge orbit (U) originates from the sphereequator (A), and in that a portion of the indentation (7) below thesphere equator (A) and a portion above the sphere equator (A) aremachined one after the other with two separate said feed movements. 2.Method according to claim 1, characterized in that,the diameter (d) ofthe cutting edge orbit plane determining the cutting edge orbit (U)connects the sphere equator (A) with a lower apex point (P) of thebearing surface (2).
 3. Method according to claim 1, characterized inthat,the bearing surface (2) has the shape of a spherical zone, and inthat the diameter (d) of the plane determining the cutting edge orbit(U) connects the sphere equator (A) with a lower limiting edge (6) ofthe spherical zone.
 4. Method according to claim 1, characterized inthat,the two feed movements are executed with the tool axis of rotation(13) being in the same disposition and extending in mutually oppositedirections.
 5. Method according to claim 1, characterized in that,thetool (14) is advanced by movement into the indentation (7) until thereis attained a disposition (L) from which the tool is moved with one ofthe two feed movements along the tool axis of rotation (13), setobliquely at the angle of inclination (α), until the tool--uponcompletion of the bearing surface region (4) below the sphere equator(A)--takes up an end position (S) in which the plane of the cutting edgeorbit (U) contains the chord (S) of the sphere having the radius (R). 6.Method according to claim 5, characterized in that,with the other of thetwo feed movements, the tool (14) is moved along the obliquely set toolaxis of rotation (13) having the angle of inclination (α) until--uponcompletion of the bearing surface region (3) above the sphere equator(A)--it takes up a further end position (S') in which the plane of thecutting edge orbit (U) contains a further chord (S') running parallel tothe first-mentioned chord (S) and having the same length which runs inthe sphere with the radius (R).
 7. Method according to claim 5,characterized in that,with the other of the two feed movements, the tool(14) is moved along the obliquely set tool axis of rotation (13) havingthe angle of inclination (α) until--upon completion of the bearingsurface region (3) above the sphere equator (A)--it takes up a furtherend position in which the plane of the cutting edge orbit (U) contains achord parallel to the first-mentioned chord (S) and of the same length,which runs in a sphere having a radius which is greater than the radius(R) of the sphere with the chord (S).
 8. Method according to claim 5,characterized in that,when the tool (14) is in the disposition (L) theintersection point (S_(p)) of the tool axis of rotation (13) with theplane of the cutting edge orbit (U) lies at the sphere centre (M). 9.Method according to claim 1, characterized in that,in addition to thefeed movement along the tool axis of rotation (13) executed during theproduction of the bearing surface portion (3) above the sphere equator(A), the tool (14) is moved in a further feed movement perpendicular tothe sphere equator (A) in a direction towards the outside of theindentation (7).